MONDAY, June 25 (HealthDay News) -- The pandemic H1N1 flu in 2009 may have killed more than 500,000 people around the world, 15 times more than reported, a new study suggests.

During the pandemic, 18,500 laboratory-confirmed deaths were reported to the World Health Organization from April 2009 through August 2010, but as many as 575,400 may have actually died, an international group of scientists now says.

"This is a better approximation of the number of deaths that occurred," said researcher Dr. Marc-Alain Widdowson from the influenza division at the U.S. Centers for Disease Control and Prevention.

"This study also confirms that the majority of deaths were in the under-65s, which is very different than seasonal influenza, where the vast majority of deaths are in the over 65s," he added.

In addition, the researchers estimate that regions in southeast Asia and Africa were more affected than the official numbers reflect, Widdowson said.

In these poorer areas, there is less ability to diagnose and treat influenza, and people in these areas aren't tested for flu, so there are far fewer reported flu deaths than actually occurred, he said.

"We need to do a better job at understanding what flu does in these settings," Widdowson said. "People in the poorer regions of south Asia and Africa get a double whammy. They may be at higher risk of severe disease and are less likely to have access to vaccines in the early part of the pandemic."

The report was published in the June 25 online edition of The Lancet Infectious Diseases.

To get a better estimate of the actual number of H1N1-related deaths, the researchers created a model drawing on data from 12 high-, middle- and low-income countries.

Believing deaths in some areas were greater than others, they weighted their model to account for these differences.

These findings are based only on those countries that collect data on the number of people who get the flu and die from it. Since in some cases the data may be sketchy, the actual number may never be known. The lack of data was mostly in middle- and low-income countries, the researchers noted.

As a rule, the number of laboratory-confirmed deaths from any disease outbreak is assumed to underestimate the actual deaths, since many people who get the disease are not tested or may not see a doctor or go to the hospital.

Applying this rule to the data, Widdowson's team estimated that the actual number of deaths from the pandemic H1N1 flu from April 2009 to August 2010 was between 151,700 and 575,400 across the globe.

Moreover, 80 percent of those who died were under 65, which is the reverse seen with usual seasonal flu, they noted.

And, they said, the flu was projected to have taken its greatest toll in southeast Asia and Africa, home to 38 percent of the world's population, where 51 percent of the deaths may have occurred.

Lone Simonsen, a research professor in the department of global health at George Washington University and co-author of an accompanying journal editorial, said, "This first estimate of the global mortality burden of the 2009 pandemic is an important study that confirms the impression that the laboratory-confirmed deaths had deeply underestimated the burden of this pandemic."

She cautioned, however, that the method used to calculate the deaths has not been used before and needs to be validated before the findings can be taken as gospel.

"Therefore, there is still uncertainty about the exact number of deaths in individual regions and countries," Simonsen added.